1. Field of the Invention
The present invention relates to electrical leads for interposers. More specifically, the present invention relates to a compliant electrical lead with a landing pad for connecting to the lead of an integrated circuit chip.
2. Description of the Related Art
Electrical interfaces between integrated circuits, e.g., microchips, and circuit board are known. Such interfaces are shown in U.S. Pat. No. 4,827,611 and U.S. Pat. No. 5,294,039. The curved interposer leads described these patents serve as electrically conductive paths between the integrated circuits and the circuit board. The curved interposer leads also form a mechanically compliant interface that withstands the damaging thermal stresses and other mechanical vibrations that degrade and destroy solder joints in previous interfaces. Compliancy is a spring-like characteristic of leads that provides some flexibility under applied pressure to absorb stress but which returns to its original orientation when the pressure is removed.
The curved interposer leads, such as those described in the above references, often come in “S” and “Z” shapes and provide a flat top “landing area” for the lead of the integrated circuit to connect with. For example, as shown in FIG. 1, the top of an S-shaped interposer lead 100 supports the base of a lead 102 of an integrated circuit chip 103, while the bottom of the S-lead 100 is supported by a circuit board 104. Solder 106 can be applied around the perimeter of the S-lead.
A drawback of these designs is that, with the continual miniaturization of electronic components and corresponding miniaturization of the compliant interposer, it is becoming more difficult to accurately place the leads of the integrated circuit on the top of the interposer lead. Also, the reduction in size also reduces the amount and thickness of solder that can be applied to connect the leads of the integrated circuit to the top of the compliant lead, which directly impacts the lifespan of the solder connection. Components must be able to withstand thermal expansion and contraction cycles over a minimum time period, but a reduction in the amount of solder reduces the number of thermal cycles that the solder can withstand before cracking. This is particularly a problem with ruggedized off the shelf (“ROTS”) components, which are in some case required with a 20-year minimum operational rating.